The basic method for the production of carbon black is well known. Generally, carbon black is produced by injecting a hydrocarbon raw material (hereinafter called "feedstock hydrocarbon") into a flow of hot oxygen-containing gas wherein the feedstock hydrocarbon is pyrolyzed and converted into a smoke before being quenched by a water spray. The hot gas is produced by burning fuel in a combustion chamber. The hot gas flows from the combustion chamber into an reaction chamber which is in open communication with the combustion chamber. The feedstock hydrocarbon is introduced into the hot gas as the hot gas flows through the reaction chamber, thereby forming a reaction mixture comprising particles of forming carbon black. The reaction mixture flows from the reactor into an exit chamber which is in open communication with the reaction chamber. At some location in the exit chamber, a quench spray is introduced into the flowing reaction mixture thereby lowering the temperature of the reaction mixture below the temperature necessary for carbon black production and halting the carbon formation reaction. The black particles are then separated from the flow of hot gas.
Conventional carbon black reactors normally comprise a cylindrical combustion chamber axially connected to one end of a cylindrical or frusto-conical reaction chamber. A reaction choke is often axially connected to the other end of the reaction chamber. The reaction choke has a diameter substantially less than the diameter of the reaction chamber and connects the reaction chamber to an exit chamber. The exit chamber is normally cylindrical and has a diameter which is substantially larger than the diameter of the reaction choke. Because the diameter of the exit chamber is substantially larger than the diameter of the reaction choke, there is an abrupt, ninety degree expansion in diameter from the reaction choke to the exit chamber.
During operation of these abrupt expansion reactors, the feedstock hydrocarbon is introduced into the flow of hot gas as the flow of hot gas passes through the reaction chamber and choke, thereby forming a flowing reaction mixture comprising hot gas and particles of forming carbon black. As the flowing reaction mixture passes from the reaction choke into the exit chamber, the flowing reaction mixture is subjected to an immediate expansion. As a result, some of the reaction mixture flows laterally towards the inner wall of the exit chamber from the reaction choke and forms eddies in the exit chamber near the outlet of the reaction choke. This effect is often referred to as "backmixing" and can cause the particles of forming carbon black to impinge against the inner wall of the exit chamber in the proximity of the reaction choke outlet.
The inner surface of the exit chamber, heated by the hot gas flowing through the exit chamber, normally has a temperature between about 3200.degree. F. to 3500.degree. F. during operation. The particles of forming carbon black which impinge on the inner surface of the exit chamber are substantially cooler than the inner surface of the exit chamber, and thus the impingement of the relatively cool particles of forming carbon black on the relatively hot inner surface of the exit chamber causes thermal shock to the exit chamber. The thermal shock erodes the inner surface of the exit chamber. The eroded portion of the exit chamber enters the flow of hot gas and results in an impurity called "refractory grit" in the carbon black product. The erosion of the exit chamber also reduces the operating life of the carbon black reactor and thus increases the cost of carbon black production.
In addition to eroding the inner surface of the exit chamber, the particles of forming carbon black which impinge on the inner surface of the exit chamber are transformed into coke by the high-temperature inner surface of the exit chamber or by rapid cooling of the particles of forming carbon black before the particles are completely transformed into carbon black. The impingement coke is a hard carbon substance with properties very different from carbon black. The impingement coke tends to enter the flow of hot gas along with the refractory grit, resulting in another impurity in the carbon black product. The presence of impurities such as refractory grit and impingement coke reduces the quality of the carbon black product and thus reduces the commercial value of the carbon black product.
Therefore, there is a need for a carbon black reactor wherein the impingement of feedstock hydrocarbon on the inner surface of the reactor is substantially reduced.